1. Field of the Invention
The present invention relates to apparatus for measuring the length of each of a series of continuously advancing articles, and more particularly provides such apparatus which is compensated for the effects of temperature variation, component drift, etc.
2. Description of the Prior Art
Various devices for measuring the length or width of moving articles are known in the art. U.S. Pat. No. 3,746,451 discloses a strip width gauge for measuring the width of an advancing strip such as magnetic tape. At each edge of the strip, a light emitting diode is positioned on one side of the strip and a photodiode on the other side of the strip. The amount of light reaching each photodiode depends upon how much light from each light emitting diode is intercepted by the strip. The light output from the photodiodes is added together to provide an indication of strip width. The disclosed strip width gauge is generally not suitable for the measurement of the lengths of advancing articles, nor is any mention made in the patent of how to modify the disclosed strip width gauge to enable the length of an advancing article to be measured.
U.S. Pat. No. 3,365,699 discloses apparatus for the automatic dimensional inspection of a moving object. The patent discloses several embodiments for inspecting the length dimension of a moving object, all of which embodiments are apparently capable of determining whether the moving object has a length within some prescribed tolerance, e.g., 1.000 .+-. 0.001 inch. None of the embodiments disclosed in the patent are capable of indicating the actual length measurement itself, e.g., 1.0007 inch.
U.S. Pat. No. 3,621,266 relates to apparatus for measuring the lengths of objects being converted with continuously conveying equipment, for example, objects such as steel slabs. The patent specifically relates to the measurement of the length of rectangular slabs when the orientation of the length axis of the slabs varies, from slab to slab, with respect to an axis defined by the direction of conveyance. To this end, disclosed apparatus performs a length measurement (generally larger than the actual length of the slab due to the obliquity of the slab relative to the direction of conveyance) and an angle measurement determinitive of the slab's obliquity. The actual length is then computed from this length and angle measurement. Both the length measurement and the angle measurement are performed by a series of spaced photocells or similar devices. Since each photocell possesses a physical size which limits the "packing density" of the photocells, the length and angle measurement are necessarily discrete measurements. When measuring steel slabs or similar objects wherein the size of the photocell is much smaller than the permissible error of measurement, there is generally no problem. However, when it is desired to perform measurements accurate to within 0.001 inch or less, the method disclosed in the patent appears less attractive.
U.S. Pat. No. 3,761,723 discloses apparatus for measuring dimensional deviations, particularly for measuring edge perforations in cine film. The disclosed apparatus employs rows of optical fibers illuminated by a light source to make such measurements. Basically, the number of fibers which are unobscured from the light source by the perforated film are counted, and knowing the size of each fiber enables the dimensional measurement to be made. Disclosed apparatus employs discrete optical fibers and thus the accuracy of measurement depends upon the size of the individual optical fibers. If it is desired to make relatively accurate measurements, a large number of tiny optical fibers are generally required. Each fiber must then have its own detector and associated circuitry.
It is desirable to make accurate and continuous length measurements of an advancing article without the use of numerous, costly or complex components. Such apparatus should preferably be capable of measuring any length within certain predetermined limits, not merely indicating that the length is within tolerance or out of tolerance. Further, it is desirable to make such length measurements rapidly, thereby enabling articles to be advanced past the apparatus at a rapid rate, as generally preferred in production operations.